Nitride semiconductor devices have been actively developed as high-withstand-voltage, high-power semiconductor devices, by utilizing their characteristics such as a high saturation electron velocity, a wide band gap, and so on. Many reports have been made on field-effect transistors, in particular, HEMT (High Electron Mobility Transistor) as the nitride semiconductor devices. Especially, an AlGaN/GaN HEMT using GaN as an electron transit layer and using AlGaN as an electron supply layer has been drawing attention. In the AlGaN/GaN HEMT, a distortion ascribable to a difference in lattice constant between GaN and AlGaN occurs in AlGaN. Owing to piezoelectric polarization caused by the distortion and to spontaneous polarization of AlGaN, a high-concentration two-dimensional electron gas (2DEG) is obtained. This makes it possible to realize a high withstand voltage and a high output power.
Patent Literature 1: Japanese Laid-open Patent Publication No. 2002-359256
In a nitride semiconductor device for high power and high frequency such as AlGaN/GaN HEMT, one of problems when it is operated under a high voltage is a current collapse phenomenon. This current collapse refers to a phenomenon that on-resistance increases by the application of a high voltage and is said to occur because electrons are trapped in semiconductor crystals, an interface between a semiconductor and an insulating film, and so on and accordingly the concentration of 2DEG in these regions decreases. As one method to suppress the current collapse phenomenon, a field plate structure has been widely known. For example, it is known that forming a gate electrode in a so-called overhanging shape has an effect to suppress the current collapse phenomenon.
However, only with the aforesaid field plate structure, it is difficult to fully suppress the current collapse phenomenon, and there is a problem that the current collapse noticeably occurs especially during a high-voltage operation, which has given rise to a demand for a method for further suppressing the current collapse.